Heretofore, a catalyst converter has been used to detoxify harmful components such as CO and NOx contained in an exhaust gas from an internal combustion engine. As the structure of the catalyst converter, a holding material is disposed between a catalyst carrier and a metal shell which covers the exterior of the catalyst carrier, and the role of the holding material is prevention of breakage of the catalyst carrier due to e.g. vibration, prevention of leakage of the exhaust gas, etc. (JP-A-11-82006). As the holding material, an alumina fiber laminate is becoming used widely with a view to dealing with the increase in the temperature of the exhaust gas to improve the rate of fuel consumption and in view of the stability during long term use, etc. (JP-A-7-286514)
In recent years, the temperature of the exhaust gas becomes increasingly high, and in DPF which is becoming used practically, collected particulates have to be destroyed by fire at a temperature of about 1,000° C. Accordingly, a holding material is required to have durability at a high temperature more than ever before. In addition, durability against wind erosion due to the exhaust gas is also required.
On the other hand, the heat resistance temperature of alumina fibers is considered to be from about 1,300 to about 1,700° C. in view of e.g. the shrinkage ratio of the fibers under heating, and it is considered that they are sufficiently practicable for use in an atmosphere of about 1,000° C. However, the present inventors have found that deterioration of fibers which is considered to be attributable to e.g. fatigue failure or creep remarkably proceeds more than expected, in a case where a compressive force is applied under heating at about 1,000° C. for a long term, such as a case of a holding material for DPF. Accordingly, even when no abnormality is confirmed externally, the restoring force (repulsive force) may be lowered due to subtle deterioration of the holding material, and accordingly e.g. a honeycomb may not be held with a sufficient repulsive force, and the exhaust gas may leak from that area. Further, due to exposure of the holding material to an exhaust gas for a long term, the holding material may be gradually eroded by wind, thus causing a gap between e.g. a honeycomb and the holding material, whereby the exhaust gas may leak or e.g. the honeycomb may slip down.
Under these circumstances, it is an object of the present invention to provide a holding material with small fiber deterioration and excellent in wind erosion resistance, even when a compressive force is applied under heating at about 1,000° C. for a long term.
Another object of the present invention is to provide an inorganic short fiber aggregate to be used for production of such a holding material and its production process.